Microwave heater

ABSTRACT

A microwave heater comprising a plurality of parallel arranged heating rectangular waveguides, each of which has a width slightly longer than that which cuts off electromagnetic waves to be used and at least one connecting waveguide member comprised of a bent waveguide and a pair of tapered waveguides fitted to both ends of said bend waveguide, thereby causing the entire assembly of waveguides to present a zig-zag form.

United States Patent [1 1 111 3,725,627 Arai [451 Apr. 3, 1973 [54] MHCROWAVE HEATER 2,560,903 7 1951 Stiefel .f'. 219 1055 463,894 8 l 69 Bl kl .219 I0. 5 [75] Inventor: Sakai Arai, Kawasaki, Japan l 9 eac 6y 5 X [73] Assignee: Tokyo Shibaura Electric Co., Ltd., Primary Kozma Kawasakpshi, Japan Assistant Examiner-Hugh D. Jaeger Attorney-R. D. Flynn et al.

[22] Filed: June 24, 1971 Appl. No; 156,434 ABSTRACT I [30] Foreign Application Priority Data June 26, 1970 Japan ..45/55372 Nov. 14, 1970 Japan ..45/l00049 Nov. 14,1970 Japan ..4S/1l2639 Nov. 14, 1970 Japan ..45/l 12640 [52] U.S. Cl ..219/l0.55 [51] Int. Cl. ..H05b 9/06 [58] Field of Search ..,.....219/10.55

[56] References Cited UNITED STATES PATENTS 3,632,945 l/l9'72 Johnson ..2l9/l0.55

A microwave heater comprising a plurality of parallel arranged heating rectangular waveguides, each of which has a width slightly longer than that which cuts off electromagnetic waves to be used and at least one connecting waveguide member comprised of a bent waveguide and a pair of tapered waveguides fitted to both ends of said bend waveguide, thereby causing the entire assembly of waveguides to present a zig-zag form.

4 Claims, 5 Drawing Figures PATENTEDAPR3 I973 SHEET 2 OF 2 FIG.4

FIGQ5 "Jul?" BACKGROUND OF THE INVENTION This invention relates to a microwave heater for performing high frequency heating using waveguides.

The known microwave heater using a waveguide is the type wherein a rectangular waveguide is bent at several points to present a winding form as a whole, with the width of said waveguide fixed. With this type of microwave heater, however, there occurs a low concentration of electromagnetic energy, so that the energy supplied is not effectively utilized with respect to objects to be heated, particularly those having a small dielectric loss. Accordingly, it has been considered necessary to use a waveguide having an increased number of bends in order to carry out full heating. This unavoidably renders such heater bulky with the resultant long heating.

SUMMARY OF THE INVENTION It is accordingly the object of this invention to provide a compact microwave heater enabling electromagnetic energy to be effectively utilized for heat treatment of objects.

According to an aspect of this invention, there is provided a microwave heater comprising a plurality of heating waveguides perforated with well aligned through holes; at least one connecting waveguide member comprised of a bent waveguide coupled and a pair of tapered waveguides to both ends of said bent waveguide so as to join adjacent heating rectangular waveguides to form a winding waveguide assembly; a power supply waveguide provided with a high frequency generator and connected to the forward end of said winding waveguide assembly so as to conduct electromagnetic energy therethrough, and a terminal waveguide containing a residual energy-absorbing element and connected to the rear end of said winding waveguide assembly. It will be noted that the width a of the heating rectangular waveguide extending in a direction perpendicular to the direction of an electric field of electromagnetic waves produced by the high frequency generator is chosen to be within the range A, a A/2, and the connecting waveguide member includes a bent waveguide having a width a within the range A 2 a a.

According to another aspect of the invention, the power supply waveguide and terminal waveguide have a larger width than the heating rectangular waveguide, are disposed in parallel or inclining relation to the heating waveguide and perforated with penetrating holes through which to conduct the object of heating.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship of a max imum field intensity E max in the heating rectangular waveguide and the length kg of electromagnetic waves passing therethrough versus the width 0 of said waveguide, with its height b and the frequency of electromagnetic waves fixed and the width a of the waveguide alone varied;

FIG. 2 is a perspective view of a microwave heater according to an embodiment of this invention;

FIG. 3 is a sectional view on line A-A of FIG. 2;

FIG. 4 is a perspective view of a microwave heater according to another embodiment of the invention; and

FIG. 5 is an apparatus for manufacturing the heater of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION There will now be described by reference to the appended drawings a microwave heater according to an embodiment of this invention. Referring to FIG. 1, the solid lines represent the results of experimentally determining the relationship of a maximum field intensity E v/cm in the heating rectangular waveguide and its width a mm, where its height b was fixed at 54.6 mm and there were used electromagnetic waves having a frequencyfof 2,450 MI-Iz (wave length A z 122 mm) with a mode of TE In said experiments there were applied l and 5 kw of power Pt(w). As seen from the graph of FIG. 1, the conventional waveguide WR 430 (RETMA specification) was unsuitable as a heating waveguide, showing that its width a should approach an electromagnetic wave cutting-off level Ac(Ac M2 61 mm). For example, where there was used 1 kw of power Pt with the width a of the heating rectangular waveguide set at 0.6 A 73 mm), then a maximum field intensity E max rose to about 245 v/cm, a far larger value than the maximum field intensity of v/cm obtained with the conventional heating waveguide WR 430. Accordingly, the heating waveguide should preferably consist of the type having a slightly larger width (for example Ac A E 0.7M than the electromagnetic wave cutting-off level Ac(=)\ /2). The aforesaid experiments used a rectangular waveguide for heating. However, there may also be used a round type. If the radius of said round waveguide is chosen to be less than 037A, in case there are used electromagnetic waves with a mode of TE and also to be less than 0.47A in case of the TM, mode, then there will be obtained a much larger maximum field intensity. The point is that the radius of said round waveguide should be chosen to be slightly larger than the electromagnetic wave cutting-off level Ac.

The broken lines of FIG. 1 indicate the relationship of the width a of the heating rectangular waveguide and the length Ag of electromagnetic waves conducted therethrough, as determined under the aforementioned conditions. As seen from the broken lines, a decrease in the width a of the heating waveguide results in an increase in the length Ag of electromagnetic waves conducted therethrough. Where, therefore, the heating waveguide is to be bent, it is necessary to choose the curvature radius of said bend to be equal to an integral multiple of one-half of said wavelength Ag so that the electromagnetic wave may not be reflected at the bent portion of the waveguide. According to this invention, however, the heating waveguide itself is not bent. There are instead used a plurality of parallel arranged heating waveguides, the adjacent ones of which are joined by a separate connected waveguide member consisting of a bent waveguide having a larger width than said heating waveguides and a pair of tapered waveguides attached to both ends of said bend waveguide, thereby causing the entire assembly of waveguides to present a winding form. Accordingly, the electromagnetic waves conducted from the heating waveguide to the bent waveguide are made to pass through said bent waveguide with a shorter wavelength, enabling the curvature radius of said bent waveguide to be fully reduced. For example,

where there are joined.

heating waveguides having a width a of 70 mm and a height b of 54.6 mm by a separate bent waveguide having the same height and width, then the curvature radius of said bent waveguide should beset at about 54 mm, with the resultant interspace of about 53 mm between adjacent heating waveguides. In contrast where the heating waveguides are joined by a bent waveguide having the same height but a much larger width of 109.2 mm, then the curvature radius of said bent waveguide will be about 42 mm to reduce the interspace between adjacent heating waveguides to about 30 mm, thus enabling a microwave heater to be made very compact.

FIG. 3 schematically illustrates a microwave heater according to an embodiment of this invention. There are parallel arranged heating rectangular waveguides l, 2 and 3. These waveguides have a width chosen to be slightly larger than that which cuts off electromagnetic waves to be used. To one end of the heating waveguide 1 is connected through a tapered waveguide 4 a power supply waveguide 6 provided with a high frequency generator for example, a magnetron. The heating waveguides 1 and 2 are joined together by a bent waveguide 9 having a larger width than said heating waveguides 1 and 2 through tapered waveguides 7 and 8 connected to both ends of said bent waveguide 9. Similarly, the heating waveguides 2 and 3 are joined together by a bent waveguide 12 having a larger width than said heating waveguides 2 and 3 through tapered waveguides 10 and 11 connected to both ends of said bent waveguide 12. Further, the heating waveguide 3 is connected through a tapered waveguide 13 to a terminal waveguide 14 provided with a residual energyabsorbing element 15. The heating waveguides 1, 2 and 3 are perforated with penetrating holes 17 through which to conduct an object of heating 16 in parallel relationship with an electric field of maximum intensity created by electromagnetic waves generated, for example, with a mode of TE by the magnetron 5. These penetrating holes 17 are made to hermetically communicate with each other by an enclosing shield 18, thereby preventing an object of heating from being exposed to the outside. I

Where the object of heating 16 is subjected to heat treatment in a microwave heater of the aforementioned arrangement, it is advisable to conduct said object 16 in the direction of the arrow 0 of FIG. 1, that is, in a direction perpendicular to that in which the heating waveguides 1, 2 and 3 are arranged in parallel. The reason is that the object of heating 16 will have a smaller dielectric loss 6 than 8 to cause the electromagnetic waves generated by the magnetron 5 to less reflected by said object 16. With this microwave heater, the heating waveguides l, 2 and 3 are connected in turn to the terminal waveguide 14 so as to satisfy their matching relationship therewith, so that the electromagnetic waves emitted by the magnetron 7 are not reflected during transmission, thus enabling substantially all of the generated electromagnetic energy to be effectively used in the heat treatment of said object 16, the residual energy being absorbed by the absorbing element disposed in the terminal waveguide 14. To prevent the leakage of electromagnetic waves through the penetrating holes 17, it is required that the first and last waveguides perforated with receiving and delivery holes be so formed as to have a relatively larger dimensions than said holes. To this end, therefore, this invention provides another type of microwave heater shown in FIG. 4 which is constructed by arranging the power supply waveguide 6 and terminal waveguide 14 both having a larger width than the intervening heating waveguides l, 2 and 3 in parallel relationship therewith, said power supply and terminal waveguides 14 and 6 being perforated with receiving and delivery holes 17b and 17a respectively.

There will now be described by reference to FIG. 5

an apparatus for manufacturing a microwave heater shown in FIG. 2. The manufacturing apparatus indicated in FIG. 5 is intended to fabricate the lower half portion of the winding waveguide assembly of the microwave heater of FIG. 2. This apparatus comprises a first bent waveguide-forming section 50 for forming the bent waveguide 12 and part of the power supply waveguide 6; first tapered waveguide-forming section 51 for forming the tapered waveguides 4, 10 and 11; a heating waveguide-forming section 52 for forming heating waveguides I, 2 and 3; a second tapered waveguide-forming section 53 for forming the tapered waveguides 7, 8 and 13, and a second bent waveguideforming section 54 for forming the bent waveguide 9 and part of the terminal waveguide 14. The heating waveguide-forming section 52 has a plurality of lengthwise extending parallel projections 55 formed on the upper surface. The interspaces between the respective projections 55 constitute a plurality of grooves 56 whose dimensions correspond to those of the heating waveguides 1, 2 and 3 to be later fabricated by being pressed thereinto. The lengthwise extending parallel projections 55 have depressions 57 to form the penetrating holes 17 and shields 18. The first and second tapered waveguide-forming sections 51 and 53 have-grooves 58 and 59 so designed as to be exactly aligned with the grooves 56 of the heating waveguideforming section 52. The first and second bent waveguide-forming sections 50 and 54 have curved grooves 60 and 61 so designed as to be exactly aligned with the corresponding ones of the grooves 58 and 59 of the first and second tapered waveguide-forming sections 51 and 53, said curved grooves 60 and 61 having a greater depth than the grooves 56 of the heating waveguide-forming section 52. The first and second bent waveguide-forming sections 50 and 54 have grooves 62 and 63 corresponding to part of the power supply waveguide 6 and terminal waveguide 14 respectively.

When there is fabricated the microwave heater of FIG. 2 using a manufacturing apparatus of the aforementioned construction, there are first fitted, for example, with screws, the first and second tapered waveguide-forming sections 51 and 53 to both ides of the heating waveguide-forming section 52, usinga and lower half portions may be welded together by proper means. Both ends of the assembly are fitted with the power supply waveguide 6 provided with the magnetron 5 and the terminal waveguide 14 containing the residual electromagnetic wave absorbing element 15, thereby finishing the microwave heater of H6. 2.

The foregoing description relates to the preferred embodiments of this invention. However, the invention is not limited thereto, but allows heating waveguides to be joined by a connecting waveguide having a length of tG/4 and a comer waveguide instead of the tapered waveguide and bent waveguide of FIG. 2. In the preceding embodiments, the power supply waveguide 6 was connected through the tapered waveguide 4 to the heating waveguide 1. However, there :may be used a power supply waveguide having the same size as said heating waveguide 1. Referring to the manufacturing aPparatus of FIG. 5, it is possible integrally to fabricate the heating waveguide-forming section 52 together with the first and second tapered waveguide-forming sections 51 and 53, or said sections 51 and 52 together with the bent waveguide-forming sections 50 and 54.

What is claimed is:

l. A microwave heater comprising:

a winding waveguide assembly including a plurality of heating waveguides perforated with well aligned penetrating holes through which to conduct an object of heating, and at least one connecting waveguide having a bent portion for joining said heating waveguides;

a power supply waveguide provided with a high frequency generator and connected to one end of said winding waveguide assembly to supply said as sembly with electromagnetic energy having a wavelength A and a terminal waveguide provided with an element for absorbing residual electromagnetic energy brought thereto without being used in heat treating said object and connected to the opposite end of said winding waveguide assembly, the width a of the heating waveguides extending in a direction perpendicular to the direction of an electric field of electromagnetic waves produced by the high frequency generator being within the range n a A /2, and the bent portion of said connecting waveguide has a width a within the range A 2 a a.

2. The microwave heater according to claim 1 wherein the terminal waveguide and power supply waveguide have a larger width than the heating waveguides, and the terminal waveguide and power a supply waveguide are disposed parallel with the heating waveguides and are perforated with penetrating holes well aligned with those of the heating waveguides to receive and deliver an object of heating therethrough.

3. The microwave heater according to claim 1 wherein all the adjacent penetrating holes communicate with each other by means of a shield.

4. The microwave heater according to claim 1 wherein the connecting waveguide comprises a bent waveguide having a larger width than the heating waveguides and a pair of tapered waveguides attached to both ends of said bent waveguide to effect connection between said bent waveguide and heating waveguides. 

1. A microwave heater comprising: a winding waveguide assembly including a plurality of heating waveguides perforated with well aligned penetrating holes through which to conduct an object of heating, and at least one connecting waveguide having a bent portion for joining said heating waveguides; a power supply waveguide provided with a high frequency generator and connected to one end of said winding waveguide assembly to supply said assembly with electromagnetic energy having a wavelength lambda 0; and a terminal waveguide provided with an element for absorbing residual electromagnetic energy brought thereto without being used in heat treating said object and connected to the opposite end of said winding waveguide assembly, the width a of the heating waveguides extending in a direction perpendicular to the direction of an electric field of electromagnetic waves produced by the high frequency generator being within the range lambda 0 > a > lambda 0/2, and the bent portion of said connecting waveguide has a width a'' within the range lambda 0 > OR = a'' > a.
 2. The microwave heater according to claim 1 wherein the terminal waveguide and power supply waveguide have a larger width than the heating waveguides, and the terminal waveguide and power supply waveguide are disposed parallel with the heating waveguides and are perforated with penetrating holes well aligned with those of the heating waveguides to receive and deliver an object of heating therethrough.
 3. The microwave heater according to claim 1 wherein all the adjacent penetrating holes communicate with each other by means of a shield.
 4. The microwave heater according to claim 1 wherein the connecting waveguide comprises a bent waveguide having a larger width than the heating waveguides and a pair of tapered waveguides attached to both ends of said bent waveguide to effect connection between said bent waveguide and heating waveguides. 